I for one hope they are introduced as bad guys, albeit because they're being manipulated or mislead, and only become Avengers at the end of the movie. Or perhaps not even join at the end of TA2, just leave things open to the possibility and have them join in a later film.

__________________"A shared universe, like any fictional construct, hinges on suspension of disbelief. When continuity is tossed away, it tatters the construct. Undermines it."
-- Peter David